Refractory for induction furnaces



Patented Dec. 27, 1938 UNITED. STATES PATENT OFFICE Henry 0. Fisher,Worcester, Mass., assignor to Norton Company, Worcester, Mass, acorporation of Massachusetts No Drawing. Application October 10, 1935,Serial No. 44,416

3 Claims.

The invention relates to refractories and with regard to its morespecific features to refractories for use in electric furnaces formelting cuprous metals.

One object of the invention is to provide an improved refractory liningfor a Wyatt induction furnace. Another object of the invention is toprovide a long life refractory lining for an induction furnace. Anotherobject of the invention is to provide an improved refractory cement.Another object of the invention is to prevent the penetration of copperinto a furnace lining in an induction type of furnace.

Another object of the invention is to provide an improved magnesiarefractory. Another ob ject of the invention is to provide an improvedaluminum oxide refractory. Another object of the invention is to providean improved method of making a preformed lining.

Another object of the invention is to provide an induction furnacecapable of satisfactorily melting pure copper or high copper contentalloys. Another object of the invention is to provide an inductionfurnace lining capable of melting pure copper or high copper contentalloys. Another object of the invention is to provide a more efiicientWyatt type of induction furnace. Another object of the invention is tomaintain, in an induction furnace, the pinch effect at a more nearlyconstant figure whereby to maintain desired cir culation of the moltenmetal. Another object of the invention is to avoid undesired decrease ofthe secondary resistance in a Wyatt type-induction electric furnace.

Another object of the invention is to provide a refractory lining whichis reducing in reaction. Another object of the invention is to provide arefractory lining of carbonaceous content. Another object of theinvention is to provide a refractory lining containing a reducingmaterial safeguarded against premature burning out. Another object ofthe invention is to impregnate a preformed lining or a fired refractorywith a reducing material. Other objects will be in part obvious or inpart pointed out hereinafter.

The invention accordingly consists in the features of construction,combinations of elements, arrangements of parts, and in the severalsteps and relation and order of each of said steps to one or more of theothers thereof, all as will be illustratively described herein, and thescope of the application of which will be indicated in the followingclaims.

As conducive to a clear understanding of the present invention referenceis hereby made to Wyatt Patent No. 1,201,761, which discloses anelectric furnace for the melting of metals and in. which the heat isgenerated in the molten metal directly by inducing therein an electriccurrent, the furnace having a primary coil disposed around a channel forthe molten metal whereby the channel, which forms a closed circuitthrough the crucible part, constitutes the secondary of the electricsystem. Such electric furnaces constructed substantially according tothe Wyatt patent of the above number, and with some variations andimprovements, have gone into extensive commercial use and are known asWyattinduction furnaces. In such a furnace circulation of the moltenmaterial is maintained by the socalled pinch and motor effects, whichare known to those skilled in this art.

In such a furnace the channel for the secondary is formed in arefractory material, and in the past a magnesia refractory has beenused. In certain cases and for certain metals an alumina refractory hasbeen used.

When melting high copper content cuprous alloys or pure cooperdifficulty has been experienced by reason of an ever increasing depositof copper in the refractory lining. Such a deposit of copper has theeffect of increasing the cross section of the secondary circuit, thuschanging its electrical properties detrimentally. This difficulty wasmost pronounced when attempts were made to melt a substantially purecopper in this type of furnace. Although pure copper has been melted ina furnace of this type with linings prior to my invention, the life ofsuch a furnace when used to melt pure copper or very high cuprous alloyshas been relatively short, and to melt copper at all in such a furnacecertain precautions and costly procedure have been heretofore used whichmay be dispensed with in the practice of my invention.

Linings for Wyatt furnaces have been generally of three types; in thegreater number of cases I the primary coils and core and the outsidelining, which might be made of metal as indicated in 4 the patentreferred to'but in many other cases included an ordinary insulatingbrick as well as the metal outer casing. were first formed and then theinner refractory lining. was formed in situ by the following procedure:Constructing a wooden core, in the desired channel shape, it was, heldin place around the primary and inthe furnace in any suitable manner. Arefractory cement was then tamped in place first outside the wooden coreand ultimately inside of it to form the refractory lining of thefurnace. The wooden core was then burned out and the refractory cementheated, as by a blow torch or otherwise, to a heat suflicient to dry itout and heat the channel to a temperature close to that of the meltingpoint of the metal to be melted in the furnace. The furnace was thenready to be run, as by the pouring of liquid metal into it and theenergization of the primary coil the fusing operation would proceed andin the use of the furnace the refractory lining would gradually becomefired.

A second method has been to form and temp entirely outside of thefurnace a refractory mixture with suitable organic temporary binder sothat the resulting shape could be handled and then insert such a liningin the furnace and proceed as heretofore stated, the final firing of thelining being due to the use of the furnace as in the first method ofprocedure.

The third heretofore used method of procedure has been to form and fireentirely outside of the furnace the lining. In both cases where thelining is preformed outside of the furnace, whether it is prefired ornot, it is only the lining for the channel that is preformed, the upperor crucible part being a refractory cement tamped into place.

The present invention may be embodied in a cement to be tamped intoplace, or it may be embodied in a preformed lining, either prefired ornot, but the method of procedure will be different in cases of apreformed prefired lining as will hereinafter appear.

I provide magnesia grain. I prefer to use a mixture of various meshsizes in order to produce a dense refractory. For example I may useone-third of grain of the mesh size from 14-24, one-third of grain ofthe mesh size from 30-90, and one-third of grain of the mesh size 100and finer.

Of such a mixture of magnesia grain I take for example parts. To this Iadd for example 4 parts of bentonite. Bentonite is considered to be theresult of devltrification and chemical alteration of the glassyparticles of volcanic ash or tuft. It is an aluminum. silicate of highadsorptive, distensive and colloidal properties which materiallypromotes workability of a mixture. It is its property of adsorbing alarge amount of water and becoming a jelly-like mass which promotesworkability. In this embodiment of the invention the bentonite alsoconstitutes the major portion of the bond. So far as many features ofthe invention are concerned I might use any other ceramic or clay ofhigh refractory properties for a bond.

I further add in the neighborhood of one part of a suitable flux.Magnesia is highly refractory, and. in order to bond it into an integralmass a flux is desirable. For my flux I prefer to use boric acid. I may,however, use other suitable fluxes such as sodium silicate.

The mixture so far specified constitutes parts. I add a suitablereducing agent to the extent of the remaining 5 parts of the Thisreducing agent is one which is such with respect to molten copper. Manysuch reducing agents might be used but I prefer to use a carboe naceoussubstance. Any carbonaceous reducing agent containing a high carboncontent is satisfactory, for example lamp black, powdered coke, powderedcoal either anthracite or bituminous, and graphite. In the preferredform of the invention I use flake graphite because it is a little lessreactive than some other forms of carbon and I desire that the reactionshall not be too violent. However, I may use substances other than thoseincluding primarily any allotroplc form of the element carbon, forexample I may use silicon carbide, boron carbide or other carbides.

When using graphite I prefer to use 5 parts in 100 as stated, but agreater or lesser amount may be used with satisfactory results. Forexample, some of the advantages of the invention may be obtained usingas low as 1% of graphite and I may use as much as 10% thereof.

The bond, which might be bentonite or some other clay, might be greatlyvaried in quantity, for example anywhere between 1% and 10%, and howmuch flux is used or whether any is used at all is a matter ofselection. I wish it to be understood that my invention is not limitedto the amount of bond or material used to promote workability.

The invention is applicable also to aluminum oxide refractory materials,but magnesia is preferred because it does not react with copper oxide.When using aluminum oxide I may use the same mesh size grain and thesame proportions of other substances mentioned.

Having thus provided a refractory mixture, I add thereto a suitabletemporary binder. While I may use any temporary binder whatsoever, forexample an organic binder such as dextrine,

Goulac or molasses, I prefer to use the inorganic I binder sodiumsilicate because of its protective qualities on the mixture during theburning out operation and subsequent use.

The refractory mixture and the temporary binder may be mixed together inany suitable manner, for example on the floor with a shovel, or in amixing machine. When using sodium silicate I use acornmercial solutionto the extent of about 1% ,or 2% thereof as sodium silicate is a fluxand I do not desire too high a fluxing action. The sodium silicate mayconstitute both the flux and the temporary binder, in which case theboric acid or the like may be omitted.

When placing my novel refractory in position in the furnace by thetamping method, I proceed according to prior practice, the tamping beingdone as usual, for example with an air hammer or the like. Then thewooden core, if such were used, is burned out in the usual manner, andthis burning out serves to dry the refractory material. Any method ofburning out, either by means of a blow torch or by means of secondarywiring imbedded in the wood, may be used.

Prior to heating the newly formed lining for the introduction of themolten metal I prefer to line the channel and also the crucible with asuitable glazing agent such as sodium silicate and silica. Such a glazemight be 75% silica, the remainder sodium silicate solution. This glazeserves to protect the graphite or other reducing substance in therefractory mixture from burning out during the heating stage just priorto the introduction of the molten copper or other alloy. If sodiumsilicate is used as a temporary binder this also has some effect inprotecting the reducing agent from burning out but I prefer also toapply the glaze for further protection.

The glazing mixture may be simply applied in any convenient manner, asby spraying or the like. In the case of sodium silicate, penetration ofair is prevented even before any firing thereof. In the cases ofpreforming the refractory, whether prefired or not, I also apply theglazing mixture, only in such cases I prefer to apply it outside as wellas inside.

For the formation of a preformed but green lining I make up therefractory mixture and temporary binder as aforesaid. For the formationof a preformed and prefired refractory lining I proceed differently. Imake up the mixture including the temporary binder without the re-'ducing agent. I then fire the mixture in a kiln in the usual manner. Ithen place the fired article in a bath of any suitable liquidcontaining, in suspension or otherwise, the reducing material. Forexample, I may use water having graphite in suspension. For example, Imay use what is known under the trade name as Aquadag, or I might usewhat is known as "Oildag. n the other hand, I may use any organicliquid, for example artificial resinous varnish (Bakelite), or molassesor the like. I then introduce the article in the bath into a vacuum boxand remove the air, for example to a vacuum of around 28", that is tosay a positive air pressure of around 2" of mercury. Having exhaustedall the air, the entrained air in the article has bubbled out throughthe liquid, and I then open up the box. As the air pressure increasesthe liquid containing the reducing agent is driven into the pores of therefractory article. In the case of organic substances such as molasses,artificial resin etc. I then proceed to carbonize the reducing agent asby heating to a suitable temperature.

In operation the reducing agent effectively prolongs the life of thefurnace and to a substantial extent prevents the growth of the secondarycircuit, either by deposit of copper or by entrance of molten copperinto the pores of the refractory. I believe that the reason why therewas copper penetration in prior refractories was partly due to thehydrostatic head of the molten metal and partly to the wetting of therefractory substance by copper oxide or oxides in the liquid state.Copper is a readily oxidizable substance, particularly in the moltenstate, and molten coppers have copper oxide in solution. Whereas copperitself does not wet a refractory, so far as I am. aware, the oxides do,or at least the copper oxide found in molten copper appears to wet therefractory. In this manner copper was carried into the refractory veryrapidly and increased the effective cross sectional area of thesecondary either as a molten copper or, in case of temperature changesand excessive penetration, as solid copper. In any case in which theliquid is frozen the copper comes out as metallic copper because copperand copper oxide do not coexist in solid solution. In many cases thepenetration of the copper with the copper oxide was extreme and extendedeven to the outside of the refractorylining or even therebeyond.

By the use of a reducing agent according to the invention, however,there is no substantial penetration by the copper in a liquid state orotherwise. I believe the beneficial result to be due to the reduction ofwhatever copper oxide comes in contact with the refractory thuspreventing the penetration as copper and also highly cuprous alloysappear not to wet a refractory substance of the nature herein described.

It will thus be seen that there has been provided by this invention amethod, a composition of matter, an article of manufacture, and anapparatus in which the various objects herein above set forth togetherwith many thoroughly practical advantages are successfully achieved. Asvarious possible embodiments might be made of the mechanical features ofthe above invention and as the art herein described might be varied invarious parts, all without departing from the scope of the invention, itis to be understood that all matter hereinbefore set forth is to beinterpreted as illustrative and not in a limiting sense.

I claim:

1. The method of making a furnace lining for cuprous induction furnaceswhich comprises mixing refractory oxide grain with a bond and atemporary binder, pressing the mixture into the shape desired, firing tovitrify the bond, placing the fired lining in a receptacle, filling thereceptacle above the level of the lining with a liquid including areducing agent which is such as to molten copper, placing the receptaclein a vacuum box, reducing the pressure, and removing the lining from thevacuum box thereafter.

2. A composition for use in the manufacture of linings for Wyatt cuprousinduction furnaces which comprises in the neighborhood of 90 parts ofgranular basic refractory oxide a substantial part of which is in grainsizes between 14 mesh size and 100 mesh size, between 1% and of a bondcomprising plastic clay having temporary binder properties, and from 2%to 10% of a reducing agent of the group consisting of carbon, graphite,silicon carbide, and boron carbide, thoroughly mixed together, therebyto form a lining which shall resist penetration of metals and metaloxides into the lining.

3. A Wyatt type of induction furnace having a refractory lining ofrefractory oxide grain and bond and comprising in the neighborhood of 90parts of granular basic refractory oxide with between 1% and 10% of abond comprising plastic clay having temporary binder properties, andfrom 2% to 10% of a reducing agent of the group consisting of carbon,graphite, silicon carbide, and boron carbide, thereby constituting alining which resists penetration of metals and metal oxides thereinto.

HENRY C. FISHER.

